Rotary pumps, such as boiler feed pumps, typically develop an axial thrust force during operation, this force resulting from the differential pressures acting across the, or each, impeller mounted on the pump rotor shaft. The pump may include a thrust bearing which supports some degree of movement of the rotor shaft. However, with high pressure pumps and/or where multiple impeller stages are provided, the axial thrust force can be significant so that some additional means of opposing the force is generally required in order to prevent axial movement of the pump rotor shaft and the resulting damage to the pump and/or pump components.
One way of countering axial movement of the shaft is to provide a balance disk assembly having a balance disk formed or mounted on the pump shaft and a stationary counter disk formed or mounted on the pump housing, the balance disk and counter disk being offset from each other. The balance disk is positioned at the high pressure/downstream end of the pump so that one side of the balance disk is exposed to the full discharge pressure of the pump. The other side of the balance disk is in fluid communication with the pump suction/inlet which is at lower pressure. Accordingly, the balance disk experiences a force equal to the product of the pressure differential across the balance disk multiplied by the exposed disk area, this force opposing the axial thrust force generated by the pump rotor shaft.
During normal operation, the pressure differential is high so that the balance disk assembly generates a force sufficient to substantially balance the axial thrust force generated by rotation of the pump rotor shaft. However, the pressure differential acting across the balance disk varies during the operational cycles of the pump. For example, where the pump rotor shaft is rotating at relatively low speed and, in particular during start-up or shut-down procedures, the opposing force may not be sufficient to balance the axial thrust force generated by the pump rotor shaft. Where this occurs, the balance disk may contact the stationary counter disk resulting in wear of the disk and can ultimately lead to significant damage to the pump and pump components, such as the pump thrust bearing for example.
Where a pump fitted with a balance disk assembly is expected to be subject to a high number of start-stop cycles, for example more than two start-stop cycles per day, it is generally recommended that a lift-off device be provided. The function of the lift-off device, as the name suggests, is to apply an axial force to the pump rotor shaft which acts to separate, or lift, the balance disk away from the counter disk and to prevent damaging contact where the opposing force generated by the balance disk assembly alone is insufficient to oppose the axial thrust force generated by the pump rotor shaft.
Even where a balance disk assembly and a lift-off device are provided, some issues remain. For example, the balance disk assembly is typically located within the pump housing and is relatively inaccessible. One result of this is that it is difficult for the pump operator to ascertain the condition of the balance disk assembly without shutting down the pump and taking the pump apart. In some cases, failure of the balance disk assembly is not identified until damage to the pump or pump components has already occurred. Also, the balance disk assembly and lift-off device represent additional components which must be taken into consideration when assessing reliability and cost effectiveness of the pump system during design, manufacture, operation, and maintenance.